1. The invention herein relates to methods for joining metals, most particularly to the attachment of lead wires to extremely thin metal films resting on deformable metal substrates.
2. It has long been a problem to accurately measure the surface temperature of certain articles, such as gas turbine airfoils, which are disposed within a stream of luminous flowing gases. Because the high heat flux gas stream is often higher in temperature than the metal can sustain, superalloy airfoils are typically internally cooled with air. Any temperature sensor and its attachment which is placed on the surface of such parts must therefore have an extremely high temperature capability. Common wire thermocouples and mechanical attachments are undesired since they can disrupt the flow of the gas stream and alter the temperature through conduction or changes in gas stream stagnation.
Using various thin film techniques, such as plating or vacuum sputtering, a thermocouple sensor can be formed by depositing layers of suitable metals upon a previously placed electrically insulating film such as a ceramic. However, until the 1970's these techniques were limited to measuring surface temperatures below about 400.degree. C. because there was a difficulty in providing a durable thin film insulating layer. Many ceramic layers which were able to be deposited either degraded and became noninsulating at high temperature; others made of deposited material such as quartz, silica, or alumina were found to be too brittle to withstand the severe thermal strains caused by the difference in coefficients of expansion between themselves and the substrate metal.
In the 1970's, coatings of the alumina-forming MCrAlY type became available. See U.S. Pat. Nos. 3,676,085, 3,754,903, and 3,928,026. It was found that careful heating of articles having such coatings could produce a thin layer of durably adherent alumina upon which thermocouple layers could be placed. Thus, thin film thermocouples of the platinum-platinum 10% rhodium type, useful to at least 900.degree. C., can be formed on MCrAlY coated articles. However, it has been a continuing problem to connect substantially larger size lead wires of similar composition to these thin conductive films. Because of the nature of a gas turbine airfoil and the factors mentioned above which discourage the use of conventional thermocouples, any connection must sustain high temperature and be very compact and strong. These requirements preclude the use of most spring devices or mechanical holders and the like. The obvious choice is for a welding process as is conventional and convenient for most connections of the sort. However, when attempted, the heat and pressure associated with such a process not only disrupts the thin metal film but also penetrates the thin and fragile insulating layer of alumina, thereby shorting out the thermocouple and destroying the desired function. Ultrasonic welding is a conventional means of joining lead wires in the semiconductor industry. However, the metals being joined there are typically soft (such as copper, gold, aluminum, etc.). No ultrasonic bonding of the Pt-Pt 10% Rh was achieved when attempted and the manufacturers of the ultrasonic machines attributed this to the hardness and other characteristics of the materials. Laser welding was evaluated but it was found that all usable welding parameters destroyed the thin insulating film.
Accordingly, there is a need for an improved method for attaching hard-high temperature alloy wires to thin metal films resting upon thin insulating layers.